#55 curve color and pos array consistency

Suppose curve.pos and curve.color both have N
entries. curve.pos = curve.pos[3:] deletes the first 3
entries of the pos array, and as a side effect the
curve.color array is adjusted to have the same length,
by deleting 3 entries from the end. At this point the
visual display is probably not what the programmer
intended. Perhaps we need a curve method that
deletes the SAME entries from both the pos and color
list, to keep them consistent with each other.